The Union Jack is one of the correct and official names for the flag of the United Kingdom since that country was formed on 1 January 1801. It combines crosses representing England, Scotland, and Ireland. An earlier form combining England and Scotland only was created in 1606. Alternative correct names for the Union Jack are the Union Flag and the Queen's Colours.

history

In 1606 England and Scotland were separate nations, though King James VI and I had ruled both since 1603. They were jointly referred to as Great Britain, and the two countries referred to as South Britain and North Britain. Neither country had a national flag. In both there was the custom of fighting and sailing under the patronage of the national saint, and flying his colours: the red upright cross on white of St George for England, and the white diagonal cross (technically called a saltire) on blue of St Andrew for Scotland.

Considering the saints' flags as the English and Scottish national flags is a modern custom.

King James was desirous of having a common emblem for his subjects to sail overseas under. In 1606 he proclaimed that all his subjects "of this Isle and Kingdome of Great Britaine" should fly the two crosses combined in their maintop, but that in their foretop should continue to fly either the red cross "commonly called St George's Cross" or the white cross "commonly called St Andrew's Cross" only, as they had formerly been wont.

That is, he ordained a jack, a specifically naval flag. The original proclamation did not give any name to the new flag. In a proclamation of 1634 it was referred to as the Union Flagge, and its use restricted to the ships of the Royal Navy. After the Restoration it was also referred to as the Union.

In 1707 England and Scotland were united into a single kingdom called Great Britain. In the part of the Proclamation mentioning the flag, it was referred to as the Union, and as Our Jack (the proclamation of course being given under the hand of Queen Anne). For the first time this ordered the use of the Union Jack as the canton of the Red Ensign, the merchant marine flag. Before this time Scots had used a form of the Union Flag with the St Andrew's cross above the St George's, but now the design was fixed, with St George's over a background of St Andrew's, as at present.

In 1801 Great Britain and Ireland were united, and to represent Ireland in a new flag, a cross was found or invented for St Patrick, being a red saltire on white. This continues to be the form used today.

The Union Jack began life as a naval jack, but Queen Anne's proclamation refers to usage on land, and it has become the national flag. Although no Royal Warrant proclaimed it to be so at a specific date, it was confirmed by the Home Secretary, Sir John Gilmor, in an answer to the House of Commons on 27 June 1933: "The Union Flag is the national flag and may properly be flown by any British subject on land."

design

Although flag designs were the business of heralds, and were officially enacted by the Sovereign in Council, since 1801 the practical business of exactly what the flag looks like has been the responsibility of the Admiralty.

Like almost all British and Commonwealth flags, the Union Jack's proportions are two to one. The various bands that constitute it have precise widths as a proportion of the whole. It is very easy to draw it wrong and quite hard to make it exactly right. The detail of the design means it is possible to fly it upside-down, though the difference is hard to spot.

Let the vertical dimension be 1. Then the horizontal dimension is 2. The whole of the topmost St George's cross, red on white, has a width of one-third. The red part of the cross is one-fifth. So each of the remaining thin strips of white around it (technically "fimbriations") is one-fifteenth.

Underneath this the combined saltires and their fimbriations have a total width of one-fifth, and the rest is the blue background. This one-fifth is divided equally between the white cross of Scotland and the red-on-white cross of Ireland. To accommodate the white background of the Irish cross, its one-tenth is divided in the proportion two to one, two-thirtieths red with a one-thirtieth white fimbriation. So the red-on-white diagonals are not even: the white diagonal above is three times wider than the one below, and the red diagonal does not pass through the middle of it.

In the first quarter of the flag, the upper one nearer the hoist (flagstaff), Scotland's white is above Ireland's red-and-white. This precedence is preserved in the other three quarters by taking the wider Scottish white as the leading edge as you go clockwise: so it is underneath in the two quarters near the fly (free-flying end), and on top again in the fourth quarter, in the lower hoist.

If these widths of the diagonals are reversed the flag is upside-down.

other countries

It is also the canton (upper left corner) of the flags of Australia, New Zealand, Tuvalu, the Fiji Islands, the Cook Islands, Hawaii, and all the Australian states, as well of course as in the flags of British dependent territories such as Bermuda. Both Tuvalu and the Cook Islands adopted new flags without it, which however proving unpopular they both restored their original designs. It was also one of the three small flags on the white stripe in the old South African flag.

It was the insipiration for the crossed-crosses flag of the united kingdom of Sweden and Norway (1807-1905), for the Basque flag, and for the new flag of Newfoundland.

My source for the details of the proclamations is Fox-Davies's book on heraldry from about 1910. This was revised by the RichmondHerald of Arms in about 1970, who added the footnote about the Home Secretary's answer in the House.

The union jack can be used as a silent alarm due to its design, with the upper red diagonals being biased towards the hoist, and the lower ones towards the fly.

Historically, if a ship was boarded by an enemy who were hoping to infiltrate a British harbour, the crew could fly the jack upside-down. The enemy would not be aware of the difference, yet the coastguard would immediately notice, and alert the appropriate authorities.